Black's move 4...Nf6 threatens White's e-pawn. White may defend it directly with either 5. d3 or 5. Nc3 but both moves interfere with White's natural plan of playing c3 and d4. Without playing c3 and d4 to create some central tension it will be difficult to generate open files for the rooks; additionally White's star attacker currently residing on a4 is liable to get swapped off if it does not have the option of retreating to c2.

A more subtle way of maintaining material equality is 5. O-O, after which Black can grab a pawn but cannot hold on to it.